1. Technical Field
This invention relates to a transfer switch which is a power switching device that automatically switches a load from a primary power source to a secondary power source upon interruption or loss of the primary power source.
In particular, this invention relates to the modification of a standard double-throw multi-pole power relay switch so that it will meet the standards of Underwriters Laboratories to qualify as a transfer switch and mitigate the problem of cross connection of power sources that results from welding of contacts that may be caused by arcing and vibration. Underwriters Laboratories Standard UL 1008, Clause 22.23 states: “The mechanism and interlocking means shall be constructed to reduce the possibility of transfer in either direction in the event of welding of one or more contacts in the power circuit.” At this time, no known commercially available model of open type multi-pole power relay complies with that requirement.
2. Description of Related Art
Systems to restore electrical power to a load in the event of source failure are in common use and necessarily include a primary and a secondary power source, and a means to switch from the primary to the secondary source in case of disruption or failure of the primary source. These systems are often complex and expensive. One example of a backup power system is illustrated by U.S. Pat. No. 5,579,197. Another such system is illustrated by applicant's prior patent, U.S. Pat. No. 6,639,330.